Biomedical research can be no better than its nutritional design. Dietary nutritional deficiencies can distort or invalidate scientific investigations. Just as important, nutrient excesses can obscure nutrient interactions or nutrient antagonists. We propose a continuous, independent evaluation and refinement of the National Research Council's listing of the nutrient requirements for the laboratory rat ("Nutrient Requirements of Laboratory Animals," 3rd Revised Edition, National ResearchCouncil, National Academy of Sciences, Washington, D.C., 1978). We propose to determine which nutrients are deficient in the published recommendations, and to reduce unnecessry nutrient excesses. It is essential to test minimal levels of all nutrients in a new formula because removal of excessive amounts of nutrients eliminates their potential sparing effect on the requirements of other interrelated nutrients. Such an improved diet should be more sensitive to changes in nutrient demand, nutrient availability, and nutrient antagonists. The improved sensitivity of this diet will be valuable in the study of the interrelationships between nutrition and disease states, toxicology and drug metabolism.